When a plurality of measuring beams is superimposed in an object space to obtain an interference pattern, this normally has the purpose to delimit an optical excitation or de-excitation in the object space to the areas of the intensity maxima of the interference pattern. In this way, a very high spatial resolution is achieved in exciting or de-exciting. In such microscopes, the required precision of the adjustment which is a pre-condition for a proper function of the microscope is very high. Actual criteria which have to be fulfilled in adjusting such microscopes include the mutual coverage of the spots of the measuring beams within a common focal plane of the objectives. With pulsed measuring beams or measuring beams of very short coherence lengths this coverage has to be ensured both spatially and temporally. Further demands with regard to the adjustment arise, if the interference pattern shall be moved for scanning a sample in the object space. Then, the phase plane of the measuring beams has additionally to be kept constant within the common focal plane of the objectives. Depending on the desired imaging mode, the relative phase of the measuring beams has to be kept constant to obtain an intensity maximum or intensity minimum always exactly in the middle of the interference pattern at the focal plane. In some cases, the phase has also to be adjusted dependently on the position in the object space to compensate for differences in the diffraction number or other particulars of a sample. If the interference pattern shall be identical over a broad wave length range of the measuring beams, even dispersion differences of the measuring beams have to be compensated for. As the wave lengths of the measuring beams used are typically smaller than 1 μm, the requirements with regard to the mechanical precision of the microscopes used are very high. Upon changing a sample, a high number of the adjustments previously made are lost. Further, thermal influences onto the microscope can neither be reduced nor reproduced to such an extent that fully constant conditions could be achieved over a longer period of time of many minutes or of even some hours. Further factors influencing an adjustment once achieved are drifts of sensors, actuators and further system components. Correspondingly, the operation of microscopes like they are for example used for 4 Pi-microscopy known from European Patent EP 491 289 B1 or in I5M-microscopy known from U.S. Pat. No. 5,671,085 require much time besides a high level of expertise.
It is a generally known aid for adjusting an optical system to direct well defined visible beams of light into the system to visualize the state of adjustment of the optical system by the visible paths of the beams of light.
In US-Patent Application published as US 2004/114225 A1 it is mentioned that a light source for generating an auxiliary path of beams may be used for adjusting the components of a microscope and that the auxiliary path of beams may be generated as an interferometric path of beams. However there is no disclosure with regard to the construction of the auxiliary path of beams and the formation of any interference pattern with this auxiliary path of beams.
Thus, there is a need for a method of adjusting a microscope and a microscope having a surveillance unit, with which the adjustment of a microscope by using auxiliary beams can be further simplified and even made automatically.